DE102012102283A1 - Method for determining aging of hydrogen fuel cells of fuel cell stack of vehicle, involves comparing difference between identified and computed catalyst carrier surface areas to measure alteration in catalyst carrier surface area - Google Patents

Abstract

The method involves determining cross over parasitic current and a short circuit resistance of a perfluoro sulfonic acid membrane (16), and computing a catalyst surface area and a catalyst carrier surface area based on a capacitance, short circuit resistance and a predetermined voltage. The difference between an identified catalyst surface area and a computed catalyst surface area is compared to measure alteration in catalyst surface area. The difference between the identified and computed catalyst carrier surface areas is compared to measure the change in the catalyst carrier surface area.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates generally to a method of quantifying the aging of membranes and electrodes in a fuel cell stack, and more particularly to a method of estimating cross-over parasitic current and short-circuit resistance of the membranes in a fuel cell stack to prevent aging of the fuel cells to determine the stack.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device including an anode and a cathode, between which an electrolyte is disposed. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and electrons in the cathode, producing water. The electrons can not pass through the electrolyte from the anode. Accordingly, they are passed over a load to perform work before they reach the cathode.

Proton exchange membrane fuel cells (PEMFCs) are popular fuel cells for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conductive membrane, such as a perfluorosulfonic acid membrane, for example. The anode and cathode typically include finely divided catalyst particles, usually platinum (Pt) dispersed on carbon particles and mixed with an ionomer. The catalyst mixture is applied to opposite sides of the membrane. The combination of the anode catalyst mixture, the cathode catalyst mixture and the membrane define a membrane electrode assembly (MEA). MEAs require adequate fuel supply and humidification for effective operation.

Typically, multiple fuel cells are combined into a fuel cell stack to generate the desired performance. The fuel cell stack receives a cathode input gas, typically with an air flow passing through the stack by means of a compressor. Not all of the oxygen is consumed by the stack, and some of the air is discharged as the cathode exhaust, and the cathode exhaust may include water as a stack waste product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.

A fuel cell stack typically includes a series of bipolar plates disposed between the plurality of MEAs in the stack, with the bipolar plates and MEAs disposed between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reaction gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates, which allow the cathode reaction gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material such as stainless steel or a conductive composite material. The end plates divert the electricity generated by the fuel cells out of the stack. The bipolar plates further include flow channels through which a coolant flows.

The MEAs are permeable, allowing nitrogen in the air to migrate from the cathode side of the stack through the MEA and collect on the anode side of the stack, often referred to as nitrogen cross-over. Although the anode-side pressure may be slightly greater than the cathode-side pressure, the cathode-side partial pressures cause oxygen and nitrogen to migrate through the membrane. The migrated oxygen burns in the presence of the anode catalyst, whereas the migrated nitrogen in the anode side of the fuel cell stack dissolves in the hydrogen. As the nitrogen concentration increases above a certain percentage, for example above 50%, fuel cells in the stack may be depleted of hydrogen. If the anode is depleted of hydrogen, the fuel cell stack may fail to produce adequate electrical power and may leak to the electrodes damage in the fuel cell stack. As the membranes age, they can become thinner, allowing nitrogen to migrate to the anode side at a faster rate.

From the prior art, it is known to arrange a vent valve at the anode exhaust outlet of the fuel cell stack to vent the nitrogen from the anode side of the stack. Further, it is known in the art to estimate the molar fraction of nitrogen in the anode side with a model calculation to determine when to deaerate the anode side or the anode subsystem. However, the model estimation can be faulty, in particular because aging causes the components of the fuel cell system, for example the membranes, to age over time. If the estimation of the anode-side nitrogen mole fraction is significantly greater than the actual nitrogen mole fraction, more anode gas is vented from the fuel cell system than necessary, that is, fuel is wasted. If the estimate of the anode-side nitrogen mole fraction is significantly lower than the current nitrogen mole fraction, then enough anode gas is not vented and there may be too few reactants in the fuel cell, which may damage the electrodes in the fuel cell stack.

Depending on the power requirements of a fuel cell system, the voltage of the fuel cell stack changes. This is known as a voltage change of the stack. A voltage change causes the catalyst particles to change, for example, the catalyst particles can aggregate to form, reducing the surface area on which the electrochemical reaction can take place. This causes inefficiencies and reduces the life of the fuel cell. In addition, the aggregation of catalyst particles can collapse the catalyst support. Corrosion of the catalyst layers can also occur, which also reduces the life of the fuel cell.

Accordingly, there is a need to determine the aging of the electrodes and membranes in a fuel cell stack over the life of the membranes in the stack in a manner that can be performed in a fuel cell stack in a vehicle without requiring the vehicle to be serviced must and without the requirement of tedious test conditions that may affect the normal operation of the vehicle. The ability to quantify electrode and membrane aging in a fuel cell vehicle presents a variety of opportunities to optimize vehicle efficiency and performance based on driving requirements.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a method for determining the aging of fuel cells in a fuel cell stack is disclosed. The method includes maintaining a constant flow of hydrogen to the anode side of the fuel cell stack, shutting off air flow to a cathode side of the stack once a predetermined concentration of hydrogen in the anode side has been achieved, and identifying a catalyst surface area and catalyst support surface area for catalyst layers in the anode fuel cell stack. The method further includes determining the total parasitic current of the fuel cell stack to determine a cross-over parasitic current and a short-circuit resistance of the fuel cell stack. The method further includes calculating the catalyst surface area and the catalyst carrier surface area of the catalyst layers and comparing the difference between the identified catalyst surface area and the calculated catalyst surface area to estimate the change in catalyst surface area.

Further features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

1 Fig. 10 is a cross-sectional view of a fuel cell;

2 is a simplified block diagram of a fuel cell system; and

3 FIG. 10 is a flowchart of an algorithm for quantifying electrode and membrane aging in a fuel cell stack.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of embodiments of the present invention directed to a method for quantifying the aging of fuel cell membranes and electrodes in a fuel cell stack over the life of the stack is merely exemplary in nature and is in no way intended to limit the invention or its applications or applications Limit uses.

1 is a cross-sectional view of a fuel cell 10 which is part of a fuel cell stack of the type mentioned below. The fuel cell 10 includes a cathode side 12 and an anode side 14 passing through a perfluorosulfonic acid membrane 16 are separated. A cathode-side diffusion media layer 20 is on the cathode side 12 arranged and a cathode side catalyst layer 22 is between the membrane 16 and the diffusion media layer 20 , arranged. Corresponding to this is an anode side diffusion media layer 24 on the anode side 14 arranged and an anode side catalyst layer 26 is between the membrane 16 and the diffusion media layer 24 arranged. The catalyst layers 22 and 26 and the membrane 16 define an MEA. The diffusion media layers 20 and 24 are porous layers that provide the transport of input gas to and the transport of water from the MEA. A cathode side flow field plate 28 is on the cathode side 12 arranged and an anode side flow field plate or bipolar plate 30 is on the anode side 14 arranged.

2 is a simplified block diagram of a fuel cell system 40 with a fuel cell stack 42 , Hydrogen gas is from a hydrogen source 44 to the anode side of the fuel cell stack 42 on the line 46 delivered. An anode exhaust gas is removed from the fuel cell stack 42 over the line 50 dissipated and in the line 46 recirculated. A bleed valve 56 is periodically opened to anode exhaust gas through a vent line 52 to vent to drain nitrogen from the anode subsystem. A pressure sensor 60 is also in the lead 50 arranged to the pressure of the anode subsystem of the fuel cell system 40 to eat. In an alternative embodiment, the pressure sensor 60 in the pipe 46 be arranged what is known to those skilled in the art.

2 FIG. 10 illustrates an example of a fuel cell system that can be used with the present invention. Other examples include fuel cell systems that use an anode flow split-split split stack design.

Air from a compressor 62 goes to the cathode side of the fuel cell stack 42 over the line 64 delivered. A cathode gas is removed from the fuel cell stack 42 on a cathode gas line 66 omitted. A mixing device 68 is in the lead 66 arranged to the cathode gas and the vented anode exhaust gas from the line 52 to mix.

A controller 54 monitors the pressure of the anode subsystem of the fuel cell system 40 , which from the pressure sensor 60 is measured, and controls the speed of the compressor 62 regulates the injection of hydrogen from the hydrogen source 44 to the anode side of the stack 42 and regulates the position of the anode vent valve 56 , which will be explained in more detail below. The controller 54 uses a model to control the migration of nitrogen from the cathode side to the anode side through the stacking membranes 16 and the concentration of nitrogen in the anode side of the stack 42 appreciate. In addition, the controller measures 54 the length of time required for the anode subsystem to reach atmospheric pressure after the fuel cell system 40 has been turned off.

The controller 54 also uses an algorithm that reduces electrode and membrane aging by quantifying the parasitic current of the membranes in the stack 42 and by quantifying the catalyst surface area and the catalyst support surface area of the catalyst layers 22 and 26 certainly. The algorithm also determines whether the parasitic current is due to gases crossing the membrane or due to short-circuit currents through the membrane, as will be explained in more detail below.

The parasitic current correlates directly to membrane function ability. When the parasitic current is low, the membranes are functional and functioning as expected. The aging of the membranes can therefore over the life of the fuel cell stack 42 be determined by using the algorithm that quantifies the above-explained parasitic current. The parasitic current for each membrane 16 in the pile 42 can be determined by measuring the voltage of each individual cell or determined by monitoring the average cell voltage and the minimum cell voltage. The Change in the parasitic current of the membranes 16 can be used to determine if rapid aging of a membrane 16 in the pile 42 occurs, or whether the membranes 16 to age more evenly.

If, based on the stacking characteristics, it is determined that the parasitic current is one or more of the membranes 16 is large enough to indicate a single event, such as a short circuit event, it may be necessary to use the fuel cell stack 42 shut down and repair. The controller 56 can indicate that a maintenance of the pile 42 is needed if a predetermined threshold for a short circuit resistance is reached. However, if the parasitic current of one or more of the membranes 16 does not indicate a short circuit event, that is the membranes 16 age uniformly, the controller can 54 the operation of the fuel cell system 10 adjust to the aging of the membranes 16 to compensate. For example, if it is determined that there is a hydrogen "cross-over", that is, the cross-over parasitic current in the membranes 16 Nitrogen can accumulate in the anode side of the stack 42 build up. Under such circumstances. the algorithm can be the controller 56 bring to the purging schedule for the anode side of the stack 42 adapt to drain the build up of nitrogen in the anode side, that is, the algorithm can cause the controller 56 adjusts the bleed schedule if a predetermined cross-over parasitic threshold is reached.

Performing accurate calculations regarding the parasitic flow of the membranes in a stack may be impractical with a fuel cell stack in a vehicle because specific test parameters are difficult to obtain and could affect the normal operation of the vehicle. However, the parasitic current can be estimated with a good degree of accuracy by performing a hydrogen takeover test, which involves maintaining a constant flow of hydrogen to the anode side and shutting off the air flow to the cathode side at a time when the hydrogen Hydrogen concentration in the anode side includes what is known to those skilled in the art. The algorithm discussed above is used to estimate the total parasitic current of the membranes in the stack, the parasitic current caused by a cross-over current, and the parasitic current caused by a short circuit current, or the short circuit resistance, during three stages of the single hydrogen takeover test. The short circuit resistance is determined from the current density, which is known to those skilled in the art. During each stage, the voltage drop of the fuel cell stack is monitored, which will be described in more detail below.

In addition to estimating the parasitic current, the algorithm further estimates the aging of the electrodes, that is, the catalyst layers 22 and 26 the fuel cells in the stack 12 , Voltage change of the fuel cell stack 12 For example, the surface area of the catalyst, which is typically platinum, may be from the catalyst layers 22 and 26 to change. For example, the catalyst of the catalyst layers 22 and 26 clump due to the voltage change. Aggregation of the catalyst particles reduces the amount of catalyst surface area and may further cause the catalyst support structure, typically carbon, to collapse. As a result, the algorithm monitors membrane aging and also the aging of the electrodes in the fuel cells of the fuel cell stack 12 ,

3 is a flowchart 70 an algorithm for quantifying electrode and membrane aging. In the box 72 For example, a hydrogen takeover test is performed, such as when the vehicle is off or in standby. Once the cathode air flow is turned off, the voltage drop of the fuel cells in the stack 42 measured. The measured voltage can be from any fuel cell 10 or from the mean cell voltage, and the cell voltage minimum can also be monitored. A predetermined drop in voltage occurs during the first stage in the box 72 measured. For example, the first 200 mV drop in stack voltage can be monitored from the open-current voltage (OCV).

Since the range of the voltage drop is relatively small, for example 200 mV, it is assumed that any short circuit resistance is constant. The capacity of the fuel cell stack 42 is known based on the amount of catalyst and catalyst carrier of the charge in the MEA. The physical volume of the cathode side of the stack 42 is also known. As a result, the amount of oxygen present in the cathode side is known, and it is also known how much current is required to supply the oxygen in the cathode side of the stack 42 consume.

berechnet, wobei I_P der gesamte parasitäre Strom, t die Zeit, C₁ der Kapazitätsfaktor für die Stufe 1, V die Spannung, β₁ der Geometriefaktor der Stufe 1, b die Tafelsteigung und γ_ORR die Sauerstoffreduktionsreaktionsordnung (oxygen reduction reaction, ORR) (0, 79) ist.Stage 1 can be defined as the stage at which the gas phase of oxygen in the cathode is largely consumed by parasitic currents. For example, the amount of catalyst surface area and catalyst carrier surface area become on the cathode side of the stack 42 in the box 74 for stage 1 between open circuit voltage and 200 mV below the open circuit voltage, and the total parasitic current of the membranes 16 in the pile 42 will be in the box 76 using the equation

where I _{p is} the total parasitic current, t is the time, C _{1 is} the capacitance factor for stage 1, V is the voltage, β _{1 is} the geometric factor of stage 1, b is the table slope, and γ _{ORR is} the oxygen reduction reaction (ORR) order. (0, 79).

The capacity factor C ₁ of level 1 can be defined as: C ₁ = (C _Ca-H + C _Ca-DL ) * rf _Ca + C _S-DL * rf _S (2)

Where C _{Ca-H is} the intrinsic catalyst hydrogen pseudo capacity, C _{Ca-DL is} the intrinsic catalyst double-layer capacity, rf _{Ca is} the roughness factor of the catalyst, C _{S-DL is} the intrinsic catalyst support double-layer capacity and rf _{S is} the roughness factor of the catalyst support.

The geometry factor β ₁ of stage 1 can be defined as:

Where F is the Faraday constant (96.485 C / mol), V _{Cath is} the total cathode volume with oxygen, A _{MEA is} the active area of the MEA, and C _{O2 is} the concentration of oxygen in the cathode.

The roughness factor rf _{Ca of} the catalyst can be defined as: rf _Ca = 10 · loading · ETA (4)

Where loading is the catalyst loading in the MEA in mg _Ca / cm ² , and ECA is the catalyst surface area in m _Ca ² / g _Ca.

Similarly, the roughness factor rf _{S of} the catalyst support can be defined as: rf _S = 10 · (loading · 100 -% Ca / 100) S _{surface area} (5)

Where S _{surface area is} the catalyst _{carrier surface area} .

The total parasitic current I _P , calculated using the algorithm in the box 76 , indicates how well the membranes 16 in the pile 42 act as insulators and as gas separators.

In addition, the algorithm calculates how much of the total parasitic current is caused by the cross-over parasitic current, that is, of gases that cause the membrane 16 during stage 3, which results in a predetermined voltage drop of the stack 42 measures. Step 3 can be considered as the step at which hydrogen adsorption occurs on the catalyst and on the catalyst support surface. The numerical values used to define each of the stages 1, 2 and 3 are merely exemplary and are not intended to limit the scope of the algorithm as described herein. For example, a drop of 300 mV after stages 1 and 2 can be observed. While the voltage drop for stage 2 is measured first, which can be defined as the stage at which most of the oxygen in the form of catalyst oxide and carrier bilayer is consumed, for example a drop of 500 mV from the stage 1 end voltage, the algorithm must monitor the voltage drop in stage 3 and calculate the "cross over" current from stage 3, since during stage 3 the short circuit resistance can be assumed to be zero, since the voltage of the stack 42 is so low. The cross-over parasitic current can be calculated according to the equation: I _XO Δt = -C ₃ ΔV + β ₃ [10 ^{-V (t) /0.035} - 10 ^{-V (t = 0) /0.035} ] (6)

Where I _{XO is} the cross-over current, C _{3 is} the capacitance factor for stage 3, and β _{3 is} the geometry factor for stage 3.

The capacity factor C ₃ for level 3 can be defined as: C ₃ = (C _Ca-H + C _Ca-DL ) * rf _Ca + C _S-DL * rf _S (7)

The geometry factor β ₃ of stage 3 can be defined as:

Thereafter, the cathode catalyst surface area and catalyst carrier surface area from stage 2 in the box 82 recalculated using the equation:

Where C _{2 is} the level 2 capacitance factor, R _{sh is} the short circuit resistance and V stage2 / t = 0 the voltage at the beginning of stage 2 is. These values will then be in the box with the assumed values 74 and numerical methods are used to iterate to an acceptable solution within a predetermined error value, for example, within a one percent error.

The capacity factor C ₂ of level 2 can be defined as follows:

The capacity term is expected to change since the pseudo-capacitance below about 600 mV is no longer discharged to reversible hydrogen electrode (RHE), that is, oxides adsorbed on the catalyst surface are completely consumed by hydrogen , The calculations are now simplified by the assumption that shorting is a function of voltage. Therefore, a relationship between short circuit resistance, crossover parasitic current, and total parasitic current can be found using the following equation:

Where I _{XO is} the "cross-over" parasitic current and V stage1 / avg the average cell voltage is during stage 1.

Thus, the stage 2 capacity of the measured voltage drop of the stage 2 is determined last, because at this time the total parasitic current, the cross-over parasitic current and the short-circuit resistance are known. As explained above, the cross-over parasitic current determined in step 3 must be determined before the short-circuit resistance is determined, as in step 3 the short-circuit current or resistance can be assumed to be zero. Therefore, the crossover parasitic current is the only unknown in step 3 and can be determined according to equation 6 discussed above.

If the difference between the calculated catalyst _{surface area} , ECA, and the assumed catalyst _{surface area} is less than a predetermined value, for example one percent, and the difference between the calculated catalyst _{carrier surface area} S _{surface area} and the assumed catalyst _{carrier surface area} is less than a predetermined value, the algorithm becomes then the calculation in the box 82 break up. If the difference is greater than a predetermined value, such as one percent, the algorithm will box the calculations discussed above 82 to repeat. Based on the difference between the calculated catalyst surface area and the assumed catalyst surface area and the difference between the calculated catalyst carrier surface area and the assumed catalyst carrier surface area, the algorithm becomes the next catalyst surface area value in box 84 estimate, that is the change in the catalyst surface area. The algorithm can indicate that maintenance of the fuel cell stack 12 is required if a predetermined threshold of the catalyst surface area is reached. The predetermined threshold depends on stacking characteristics.

The foregoing discussion discloses and describes purely exemplary embodiments of the present invention. One skilled in the art can readily appreciate from the specification and from the appended drawings and claims that various changes, modifications and variations can be made without departing from the spirit and scope of the present invention as defined in the following claims.

Claims (10)

A method of determining the aging of fuel cells in a fuel cell stack, the method comprising: Maintaining a constant flow of hydrogen to an anode side of the fuel cell stack and shutting off air flow to a cathode side of the fuel cell stack once a predetermined concentration of hydrogen has been reached in the anode side; Identifying a predetermined voltage drop of the fuel cell stack after switching off the air flow to the cathode side; - identifying a catalyst surface area and a catalyst carrier surface area of catalyst layers in the fuel cell stack; Determining a total parasitic flow of membranes in the fuel cell stack; Determining a crossover parasitic current and a short circuit resistance of the membranes from the total parasitic current; Calculating the catalyst surface area and the catalyst carrier surface area based on a capacity factor, the short circuit resistance and a predetermined voltage; Comparing the difference between the identified catalyst surface area and the calculated catalyst surface area to estimate the change in catalyst surface area; and Comparing the difference between the identified catalyst carrier surface area and the calculated catalyst carrier surface area to estimate the change in the catalyst carrier surface area. The method of claim 1, wherein identifying the predetermined voltage drop of the fuel cell stack after switching off the air flow to the cathode side includes identifying the voltage drop of the fuel cell stack in three stages, each stage being a predetermined voltage drop after switching off the air flow to the cathode side. The method of claim 2, wherein a first stage of the three stages includes measuring approximately a 200 mV voltage drop from an open circuit voltage. The method of claim 3, wherein a second stage of the three stages includes measuring approximately a 500 mV voltage drop from a first stage end voltage. The method of claim 4, wherein a third stage of the three stages includes measuring approximately a 300 mV voltage drop from a second stage end voltage. The method of claim 3, wherein the total parasitic current of the first stage is determined after the first stage voltage drop is complete. The method of claim 5, wherein the cross-over parasitic current is determined after the third-stage voltage drop is complete. The method of claim 4, wherein the short circuit resistance is determined after the second stage voltage drop is complete. The method of claim 4, wherein the catalyst surface area and catalyst carrier surface area are calculated after the second stage is complete. The method of claim 1, wherein comparing the difference between the identified catalyst surface area and the calculated catalyst surface area and the identified catalyst carrier surface area and the calculated catalyst carrier surface area includes iterating to an acceptable solution within a predetermined error value.

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